In January of 1957 he made his NHL debut with Boston when he was purchased from Springfield of the AHL. The Bruins Terry Sawchuk would be out for the remainder of the year with "nervous exhaustion". 25 year old Simmons filled in admirably going 13-9-4 with 2.42 GAA. He then lead them past the first place Red Wings before losing four games to one to Montreal in the final.
The following year Simmons shared the net duties with Harry Lumley and once again led the B's to the finals. His 2.41 regular season GAA was third in the circuit and he improved to 2.24 in 11 post season tilts. 1958/59 was Smmons only season as undisputed starter. playing 58 games. However by the time the playoff rolled around, Harry Lumley had usurped Simmons as starter and played every game in a first round loss to Toronto.
In January of 1961 Simmons was traded to the Leafs and served as Johnny Bower's main back-up for three years. In 1962/63 he actually led the NHL with a 2.46 GAA while playing 28 games.
It was in the spring of 1962 when Don Simmons proved instrumental in Toronto's first Stanley Cup victory in eleven years. He had played a mere 9 games in relief of the "China Wall" Bower but had to step into game four of the finals when the old vet went down with a leg injury.
Simmons played the final 45 minutes of a 4-1 loss to Chicago after Bower was injured making a save on Bobby Hull. He was in civilian clothes in the stands when summoned to fill in, and the final five minutes of the first were added onto the start of the second allowing Simmons time to get dressed. When the series returned to Toronto for game five there was thoughts of freezing Bower's leg to get him between the pipes,but Simmons was called again and stopped 26 of 30 shots in 8-4 Leaf win.
In game six Simmons earned his keep in stopping 20 of 21 shots allowing only a 3rd period Bobby Hull marker. Don Simmons finished the three finals games with two wins, one loss and an average of 2.91, and was the man in charge of the twine-tent when the Leaf's "curse of Barilko" was broken.
That would be Simmons' last ever NHL playoff match as he would play bits of three seasons with the Rangers backing up Ed Giacomin. Simmons would post years of 35 and 38 victories in the minors before retiring at the age of 37. Born in Port Colborne, Ont in 1931, Don Simmons was 69 years old.